One study, published in JAMA Pediatrics, studied 132,000 births in Vancouver, Canada, from 2004 to 2009. Researchers concluded there was a link between exposure to nitric oxide from car exhaust during pregnancy and greater incidence of childhood ASD.

The second study, published in Environmental Epidemiology, observed more than 15,000 infants born in Denmark between 1989 and 2013. It found that air pollution exposure during the first months of life and later was also associated with ASD.

“The study showed a small increase in autism for infants exposed before birth to one of the pollutants: nitric oxide. While it’s a small increase, if large populations are exposed, it could still affect many children,” Lynn Singer, PhD, professor of population and quantitative health sciences, pediatrics, psychiatry, and psychology at the School of Medicine at Case Western Reserve University in Ohio, told Healthline.

“It confirms some previous research and suggests that air pollution should be studied further regarding how it relates to autism,” she added.

However, neither study proved that air pollution is causing ASD.

The researchers have only found that children in an area with air pollution are at a greater risk.

Research that was originally published in the Journal of Inorganic Biochemistry had found that aluminum in certain vaccines (used to cause a stronger immune response) actually caused ASD in mice.

Then, other scientists took a close look at the study and found major problems with how the researchers came to this conclusion. There was even evidence that some of the data may have been faked.

The study was eventually retracted.

Another investigation, involving only 12 children, was published in 1998. The study stated that symptoms of autism emerged soon after the children had received the MMR (measles, mumps, and rubella) vaccine.

The blame was placed on thimerosal (a form of mercury) in the vaccine.

However, the results of several large U.S., European, and Japanese studies show that, although the rate of MMR vaccination has remained constant or declined, the rate of children diagnosed with autism has soared.

The Danish government even ceased using vaccines containing thimerosal, but ASD rates continued to rise.

“The theory that vaccines cause autism has been thoroughly disproved in the scientific literature,” confirmed Singer.

According to the Centers for Disease Control and Prevention (CDC) diagnosing ASD can be difficult because “there is no medical test, like a blood test, to diagnose the disorders. Doctors look at the child’s behavior and development to make a diagnosis.”

So doctors rely on established medical guidelines to diagnose ASD.

But, with every new edition of the Diagnostic and Statistical Manual (DSM) of the American Psychiatric Association and the International Classification of Diseases of the World Health Organization, the criteria for diagnosing ASD have expanded.

ASD was changed from a severe disorder to a condition that includes mild forms.

Also, there’s improved detection of ASD, in adults as well as children.

Asked whether this could increase the rate of ASD being diagnosed, Singer said “I can’t determine exactly, but certainly changing and expanding diagnostic criteria is a factor, as is increased awareness and policy changes.”

Scientists are working to create accurate medical tests that can tell whether someone has this disorder.

Recently published research from Stanford University found that low levels of the hormone vasopressin in a child’s spinal fluid might predict their odds of developing ASD.

Scientists from the University of Warwick in England have developed a diagnostic test using blood and urine that might predict ASD with 92 percent accuracy.

“Most children with developmental delay or disabilities have an unknown cause except for a few that can now be identified, like Down syndrome. Identification of a specific medical diagnosis will eventually lead to prevention or a cure,” said Singer.